In a conventional halogen lighting system as schematically and exemplarily illustrated in the upper part of FIG. 1, a halogen bulb 2 or a plurality of halogen bulbs is/are connected via an AC/AC transformer 4 to a mains supply having a voltage of typically 230 V at 50 Hz in Europe and 110 V at 60 Hz in the U.S. If a classical iron core transformer 4 is used, the turns ratio is adapted to the mains voltage so as to provide 12 V AC to the halogen bulb 2. Since classical transformers are large and heavy due to the iron core and copper windings, so-called electrical transformers which are small primary switched AC/AC converters are preferably used in modern halogen systems.
To vary the brightness of the halogen bulb 2, a TRIAC dimmer 6 is placed on the primary side of the transformer 4. The TRIAC dimmer 6 cuts the leading or trailing edge (depending on the type of the transformer) of the sinusoidal mains voltage as illustrated by the inset a) and b) in FIG. 1. Consequently, the RMS input voltage at the transformer 4 is reduced. This reduced voltage is subsequently transferred to the secondary side of the transformer 4 where it is applied to the halogen bulb 2. As halogen bulbs are almost ideal resistive loads, the RMS power is reduced and thereby the brightness of the halogen bulb 2. Alternatively, the TRIAC dimmer may be placed on the secondary side of the transformer 4, the functionality is the same as already explained.
Light emitting semiconductor devices, especially light emitting diodes (LEDs) are more and more used in lighting systems due to their low power consumption and low operating temperature. A schematic LED system is shown in the lower part of FIG. 1. An AC/AC transformer 4 is connected to the mains supply and since the LEDs 8 need a direct current, an additional LED driver 10 is placed on the secondary side of the transformer 4. A typical LED driver 10 comprises a current mode boost controller for driving a switched voltage converter, e.g. a boost-, buck- or sepic-converter (single ended primary inductance converter). A suitable controller 10 is e.g. the TPS40210 or TPS40211 controllers from Texas Instruments. The LED driver is configured to supply a constant current to the LEDs 8. This current is in general independent from the input and output voltage since an LED is controlled by current and not by voltage. The controller adjusts the duty cycle of the switched voltage converter in such a way that the output current of the LED driver 10 remains at a predetermined fixed value.
To provide the LED lighting system with a dimming capability, an external dimming signal DIM is needed. This dimming signal DIM has to be generated by an additional circuit, e.g. a dimming microcontroller 12, and has to be coupled to a feedback line of the LED driver 10. Typically, a high frequency (e.g. 1 kHz) is applied to the LED driver 10 and for dimming purpose. The LED is switched on and off using the aforementioned high frequency to prevent any flickering which could be seen by human eyes. Typically, when the dimming signal DIM is in a high state, the voltage seen by the LED driver 10 on a feedback pin is higher than an internal reference voltage and accordingly the LED driver 10 stops switching. The output current provided to the LEDs 8 is zero and the LED is not working anymore. If the dimming signal DIM is in low state, the LED driver 10 is not influenced and accordingly it provides a constant current to the LEDs 8. Depending on the duty cycle of the applied dimming signal DIM, the average output current of the LED driver 10 and accordingly the luminance of the LEDs 8 is changed. However, there is a need for a dimming signal DIM. Consequently, a dimming microcontroller and an additional line is necessary too.